I've seen parts costs of around $500 to add power steering to the B7100( and B6100 ), several others here have done it. Search for "power steering b7100." If it wasn't a factory option on that model (or another L-series that is basically the same as the L2550, but with a different engine, etc) then I'm sure your L2550 would be a similar job.
In the later case, you might be able to just buy all the correct parts to change it over, but you might be surprised by the cost of doing do.
I was considering the conversion for my own B7100. I opted, as you apparently did, to buy another tractor that has power steering. Surprisingly, when the dust settled, the upgrade cost less than adding power steering to my B7100 would have. I did buy used, and waited along time for the right deal.
I think you just need the right buyer. Not every buyer cares about power steering -- or even thinks about it, until operating an FEL w/o it fir a while.
Also, a major modification like this may even scare off some buyers.
I, personally would rather buy one that came from the factory with power steering, than one with a custom power steering system (assuming that it can't be done with factory parts).
So, the short answer is, I would think that the difference in the price you can get might not be worth the cost and labor of the conversion. I would only see the conversion worth while if you wanted to keep the tractor.